Electron discharge apparatus



July 9, 1940. J. R. PIERCE ELECTRON DISCHARGE APPARATUS Filed May 4,1938 VAR/ABLE L lG/v' 7' SOURCE DIRECTION OF DIRECTION OF LIGHT BEAMELECTRON STREAMS //Vl/EN7'0R By J. R. P/ERCE Malia/QM A T TOR/VEVPatented July 9, 1940 UNITED STATES ELECTRON DISCHARGE APPARATUS John R.Pierce, New York, N. Y., assignor' to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Application May4, 1938, Serial No. 205,928

11 Claims.

This invention relates to electron discharge apparatus and moreparticularly to such apparatus including an electron discharge devicehaving a pair of cooperating secondary electron emitting electrodes andcommonly designated as electron multipliers.

One object of this invention is to facilitate the manifold amplificationof electrical or light impulses.

Another object of this invention is to improve the efficiency andoperating stability of high frequency electron multipliers.

Another object of this invention is to obtain substantially equalelectron transit times for a multiplicity of electron streams flowingbetween a pair of electrodes in an electron multiplier.

Another object of this invention is to minimize or prevent space chargeeffects in electron multipliers.

A further object of this invention is to simplify the structure ofelectron multipliers.

Still another object of this invention is to increase the power capacityof electron multipliers.

In one illustrative embodiment of this invention, an electron multipliercomprises a pair of cathodes having opposed secondary electron emissivesurfaces and a collector electrode or anode disposed adjacent one end ofthe cathodes. During operation of the multiplier, one of the surfaces isenergized at the end thereof remote from the collector electrode oranode to cause emission of electrons therefrom. These electrons flowtoward the opposite emissive surface and impinge thereon to cause theemission of secondary electrons therefrom. These secondary electrons, inturn, flow to the first surface and by impinging thereon cause theemission of other secondary electrons. This action is repeated along thetwo emissive surfaces, toward the collector electrode or anode, and thesecondary electrons emanating from the end of one surface nearest thecollector electrode flow thereto and constitute the output current.

Because of the character of the emissive surfaces, each impingingelectron results in the release or emanation of a plurality of secondaryelectrons so that repeated electron multiplications of the original orprimary electron current, and amplification of the signal or impulsecorresponding thereto are obtained.

In accordance with one feature of this invention, the cathodes are soshaped that substantially all of the electrons emanating from eachemissive surface are confined between these surfaces and are guided orfocussed so that they impinge upon the opposite surface. In oneillustrative form, the cathodes may be opposed halves of frustrums ofcones or pyramids, the larger ends thereof being toward the collectorelectrode or anode. 5'

In accordance with another feature of this invention, means are providedfor producing strong fields between the emissive surfaces so that theelectrons emanating from each surface are subjected to acceleratingfields and caused to im- 10 pinge at high velocities upon the othersurface. This means enables the attainment of very low and uniformtransit times and voltage saturation of the electrons. In oneillustrative form, the accelerating electrodes may be diametrically op-15 posite plate members extending between adjacent longitudinal edges ofthe cathodes.

The invention and the foregoing and other features thereof will beunderstood more clearly and fully from the following detaileddescription with 20 reference to the accompanying drawing in which:

Fig. 1 is a perspective view of an electron multiplier illustrative ofone embodiment of this invention, portions of the enclosing vessel beingbroken away to show details more clearly;

Figs. 2 and 3 are side views in elevation and at positions at rightangles to each other-0f the electrodes of the electron multiplier shownin Fig. 1, illustrating the form and space relation thereof;

Fig. 4 is a view in section along line 4-4 of Fig. 2;

Fig. 5 is a detail perspective View of one of the cathodes embodied inthe multiplier shown in Fig. 1; 35

Fig. 6 is another perspective View illustrating a modification of thecathode shown in Fig. 5; and

Figs. 7 and 8 are circuit diagrams illustrating typical ways in which anelectron multiplier constructed in accordance with this invention, maybe operated.

Referring now to the drawing, the electron multiplier shown in Fig. 1comprises a highly evacuated enclosing vessel iii having an inwardlyextending stem II at one end, terminating in a 45 press l2. Mountedabove the stem are a pair of cathodes I31; and l3b, an anode orcollector electrode I l and a pair of auxiliary, accelerating or fieldelectrodes I51; and I517.

The cathodes I3 may be formed of sheet metal, for example sheet silver,and the opposed surfaces thereof are treated or coated to form a coatingor layer having good secondary electron emitting properties. Forexample, these surw faces may be treated to form thereon a matrixincluding silver, caesium oxide and some free caesium. These surfaces,furthermore, are elongated and channel shaped, increasing in width andspacing toward the collector electrode or anode I4. For example, asshown clearly in Figs. 3 and 5, each of the cathodes may besubstantially one-half of a right frustum of a cone, the larger basebeing toward the collector electrode or anode. Preferably, as shown inFig. 4, the cathodes I3 are concentric with each other.

The cathodes I3 may be mounted individually by rigid metallic supportsor uprights I6 aflixed to and extending from leading-in conductors I1,which are embedded in the press I2 and project through the stem I I.

The anode or collector electrode I4 may be a metallic plate or disc,preferably coaxial with the axis of concentricity of the cathodes I3,supported by a pair of rigid metallic uprights or supports I8 arisingfrom wires I9 sealed in the press I2. Electrical connection to thecollector electrode or anode may be established through a leading-inconductor 20 connected to one of the wires I9.

The auxiliary electrodes I50, and I517 may be metallic coplanar plates,extending diametrically opposite with respect to the cathodes l3 andbetween the juxtaposed longitudinal edges thereof. These electrodes I 5aand I51; may be supported by rigid rods or wires 2I extending frommetallic stubs 22, which are embedded in the press I2 and haveleading-in conductors 23 connected thereto. Preferably, as illustratedin Fig. 2, the inner edges of the auxiliary electrodes are somewhatcurved and equally spaced on opposite sides of the axis of concentricityof the cathodes During operation of the electron multiplier, asillustrated in Figs. 7 and 8, the anode or collector electrode I4 may bemaintained at a positive potential, for example of the order of 200volts, with respect to the two cathodes I3, as by a source 24, such as abattery, in series with an output device or circuit 25, and theauxiliary electrodes I5a and I5b likewise may be maintained at apositive potential, for example, of the order 013200 volts, with respectto the cathodes I3, as by a source 26, such as a battery. Means areprovided for producing a high frequency field between the opposed orconcave surfaces of the cathodes I3.

As illustrated in Fig. 7, this high frequency field may beobtained-"through an oscillator 21 coupled between the cathodes I3 tiedtogether and the auxiliary electrodes tied together through ananti-resonant circuit including a condenser 28 and inductance 29connected in parallel.

Alternatively, as illustrated in Fig. 8, the oscillator 2! may becoupled to the anti-resonant circuit 28, 29 connected directly betweenthe cathodes I3a and I3b.

Preferably, the frequency of the oscillator is such that in thearrangement shown in Fig. '7,

and in the arrangement shown in Fig. 8,

where j is the frequency and T is the electron transit time between thetwo cathodes I3a and I3b.

When a portion of the dished or concave surface at one end of one of thecathodes, for example, the lower end of the cathode I3a in Fig. 3, isenergized, as by a beam of light focussed thereon from a variable lightsource 30, primary or photoelectrons are emitted. These electrons comeunder the influence of two fields, namely, a strong field produced bythe auxiliary electrodes I5a and I51), which accelerates the electronsaway from the cathode I 3a and toward the dished or concave surface ofthe cathode I31), and a high frequency field accelerating the electronsin the direction of the collector electrode or anode I4. The primary orphotoelectrons will traverse paths as indicated generally by the arrowsin Fig. 4 and impinge upon a portion of the concave surface of thecathode I3b spaced from the end thereof remote from the anode orcollector electrode I4. The impinging electrons will cause the emanationof secondary electrons from this portion of the cathode I3b, thesecondary electron current being several times as great as the primaryelectron current because of the treatment or coating of the concavesurfaces of the cathodes as heretofore described. These secondaryelectrons under the influence of the high frequency field and theaccelerating field created by the auxiliary electrodes I5 flow to thecathode I3a, impinge thereon at an area displaced toward the collectorelectrode with respect to the area of emanation of the primaryelectrons, and thereby cause the emission of a still greater secondaryelectron current from the cathode I3a. This action is repeated along thecathodes I3a and I3b, toward the anode or collector electrode, theelectron trajectories being indicated generally by the arrows in Fig. 3,and the electrons emanating at the end adjacent the anode or collectorelectrode fiow thereto and constitute the output current. Because of therepeated electron multiplications at the cathodes, the output currentwill be much greater than the primary electron current and hencerepresents a manifold amplification of the signal corresponding to thelight beam produced at the source 30.

Because of the inclination of the emissive surfaces of the cathode andthe direction of the fields thereadjacent, the electrons emanating fromeach portion of these surfaces will be directed toward a portion of theopposite surface nearer the collector electrode'or anode. Inasmuch asthese surfaces are channel shaped, concave or dished, the variouselectron streams will be substantially confined between these surfacesand the streams originating at each surface will be focussed upon arestricted portion or area of the opposite surface. Hence, concentrationof the electron streams is achieved and the attainment of a highefiiciency enabled.

The auxiliary electrodes, as noted heretofore, provide strong fieldsaccelerating the electrons emanating from each of the electron emissivesurfaces and thereby insure relatively short electron transit times and,in addition, assist in the focussing of the electron streams.Furthermore, the strong fields assure acceleration of substantially allelectrons away from the surface at which they originate, prevent spacecharge effects and allow voltage saturation of the electron streams.

The effect of the increase in spacing between the emissive surfaces ofthe cathodes, toward the collector electrode or anode, is counteractedby the increased extent to which the auxiliary e1ectrodes extend betweenthe two cathodes so that substantially equal electron transit times areobtained along all the paths followed by the electrons flowing betweenthe two cathodes.

As will benoted, particularly from Fig. 2, the extent to which theauxiliary electrodes I50 and use project between the two cathodes I3increases toward the collector electrode or anode so that consequentlythe fields increase in intensity in the same direction in which thespace current increases, that is, toward the collector electrode. Hence,space charge effects in the vicinity of the collector electrode or anodeare substantially minimized and undue restriction of the output currentof the multiplier is prevented.

The fields may be varied or adjusted as desired by altering the distancebetween the opposed edges of the auxiliary electrodes, by altering thedistance to which these electrodes project toward the axis of theelectrode structure or by altering both of these distances.

It may be remarked that in electron multipliers constructed inaccordance with this invention, the various electron streams areconcentrated solely by the fields produced by and between the electrodesso that the use of external means, such as magnets or field coils, isobviated, and,

thereby, the construction of electron multiplying apparatus is greatlysimplified and. its cost reduced.

It may be remarked also that in electron multipliers constructed inaccordance with this invention, because of the configuration and spacerelation of the emissive surfaces, the electrons emanating from anysmall area in one surface are prevented from returning to the same areaso that the establishment of a space charge condition or electronregeneration effect is precluded. Hence, a large increase in outputcurrent substantially independently of an increase in the original orprimary current occasioned by the light beam is avoided and stableoperation of the multiplier is attained.

Although specific embodiments of this invention have been shown anddescribed, it will be understood that they are but illustrative of thisinvention. For example, although the cathodes it have been shown anddescribed as semifrustro-cones, they may be of other configuration, suchas semi-frustro-pyramids as illus" trated in Fig. 6. Also, although thecathodes have been shown as having linear longitudinal elements andconstant lateral curvature, they may be curved somewhat longitudinally,and the curvature of successive or spaced lateral sections may bechanged to improve or alter the degree of convergence and focussing ofthe various electron streams. Various other modifications may be made inthe structures and circuits disclosed without departing from the scopeand spirit of this invention as defined in the appended claims.

Reference is made of the application Serial No. 227,649, filed August31, 1938, of William Shockley, disclosing a related invention.

What is claimed is:

1. An electron multiplier comprisinga pair of opposed channel-shapedcathodes, and a collector electrode at one end of said cathodes, thespacing between the opposed surfaces of said cathodes increasing towardsaid collector electrode. I 2. An electron multiplier comprising a pairof cathodes having elongated opposed electron emissive surfaces andlongitudinal edges, a collector electrode at one end of said surfaces,and auxiliary electrodes adjacent the longitudinal edges of saidcathodes and substantially coextensive therewith.

3. An electron multiplier comprising a pair of opposed electron emissivesurfaces having juxtaposed longitudinal edges, the spacing between saidsurfaces increasing toward one end thereof, a collector electrodeadjacent said end, and a field electrode extending between juxtaposedlongitudinal edges of said surfaces.

4. An electron multiplier in accordance with claim 3 wherein thedistance to which said field electrode extends inwardly from said edgesincreases toward said one end'of said surfaces.

5. An electron multiplier comprising a pair of cathodes having opposedelongated concave emissive surfaces increasing in lateral section fromone end to the other and having juxtaposed longitudinal edges, and acollector electrode adjacent one end of said surfaces.

6. An electron multiplier in accordance with claim 5 comprising a pairof auxiliary electrodes adjacent the longitudinal edges of saidsurfaces.

7. An electron multiplier comprising a pair of elongated concentric,concave emissive surfaces, a collector electrode at one end of saidsurfaces, and substantially coplanar plate electrodes extending betweenadjacent edges of said surfaces and substantially coextensive therewith.

8. An electron multiplier comprising a pair of electrically separate,substantially semi-conical electron emissive surfaces facing each otherand having opposed, spaced longitudinal'edges, and a collector electrodeopposite the base of said surl faces.

9. An electron multiplier in accordance with claim 8 comprising a pairof plate auxiliary electrodes extending between the opposed longitudinaledges of said surfaces.

10. An electron multiplier comprising a pair of cathodes having opposed,concentric, substantially semi-conical electron emissive surfaces andjuxtaposed longitudinal edges, a pair of diametrically opposite coplanarplate electrodes extending between said edges, and a collector electrodeopposite the base of said surfaces.

11. An electron multiplier in accordance with claim 10 wherein the inneredges of said plate electrodes are curved and equally spaced on oppositesides of the axis of concentricity of said surfaces.

JOHN R. PIERCE.

